/Engine/Plugins/Runtime/OpenXR/Source/OpenXRHMD/Public/IOpenXRExtensionPlugin.h
Experimental: Optionally hand off the loader/plugin GetInstanceProcAddr to an extension plugin to support API layering. Returns true if plugin is wrapping API. Layers can chain by using received GetProcAddr to hand off API calls.
virtual bool InsertOpenXRAPILayer ( PFN_xrGetInstanceProcAddr & InOutGetProcAddr )